1. Application Fields of the Invention
The present invention relates to an apparatus and method for monitoring fouling resistance and cleanliness factor of a heat-transfer surface of a heat exchanger, and in particular, to an improved system for monitoring the precise fouling tendency on a heat-transfer surface of a heat exchanger and a method thereof which are capable of continuously and accurately observing scale deposition in a heat exchanger due to hardness components and suspended solids contained in cooling water by measuring an average temperature on the heat-transfer surface of a heat exchanger.
2. Description of the Conventional Art
A plurality of heat exchangers are used in cooling systems of power plants, and petro-chemical plants. Since there are various impurities in cooling water such as dusts, suspended solids, microorganisms, and metal oxides, when cooling water containing impurities flows in the heat exchanger, scales and deposits are usually formed on a high temperature heat-transfer surface or in a low flow rate zone of the heat exchanger. The thusly-formed scales cause a decrease in heat-transfer efficiency of the heat exchanger and increase the flow resistance of a liquid.
Scale components which cause deposit formation on a heat-transfer surface of a heat exchanger include minerals such as calcite, whitlockits, gypsum, sepiolite, iron oxides, silica, etc. These scales are produced by the following chemical reactions.
i) Calcite EQU Ca.sup.+2 +2HCO.sup.-.sub.3 .fwdarw.CaCO.sub.3 +CO.sub.2 +H.sub.2 O
ii) Hydroxyapatite/whitlockits EQU 5Ca.sup.+2 +3HPO.sup.-2.sub.4 +4OH.sup.- .fwdarw.3H.sub.2 O+Ca.sub.5 (PO.sub.4).sub.3 OH
iii) Gypsum EQU Ca.sup.+2 +SO.sup.-2.sub.4 +2H.sub.2 O.fwdarw.CaSO.sub.4.2H.sub.2 O
iv) Sepiolite EQU 2Mg.sup.+2 +3Si(OH).sub.4 +4OH.sup.- .fwdarw.4.5H.sub.2 O+Mg.sub.2 Si.sub.3 O.sub.7.5 (OH).3H.sub.2 O
As shown in Table 1, since the above-described scale components have relatively low heat-transfer coefficients compared to metals, when such scale components deposit on the heat-transfer surface of the heat exchanger, the heat-transfer efficiency of the heat exchanger decreases.
TABLE 1 ______________________________________ heat-transfer coefficients of typical metals and scales Classification Thermal transfer coefficient ______________________________________ Metal Al-Brass 700 Cu--Ni (70:30) 310 Scale Calcite 3 Whitlockits 26 Gypsum 18 Oxidized steel 7 ______________________________________
Therefore, in order to prevent the efficiency of the heat exchanger from decreasing due to scale deposits on the heat-transfer surface of the heat exchanger, a fouling monitoring system is used. The water treatment method of a cooling system can be improved based on the thusly-measured results which are obtained by the fouling monitor, and a proper cleaning period of the heat exchanger tube and an efficiency of the heat exchanger are determined.
In addition, Table 2 illustrates the operation values of fouling resistances of the heat exchanger which is generally applied in the industry. As shown therein, the fouling resistance coefficients are different in accordance with the kinds of liquids which pass through the heat exchanger. Actually, during the operation of the heat exchanger, the heat exchanger should be operated within the permitted fouling resistance values thereof so that the heat exchanger is not fouled excessively, whereby it is possible to obtain the design efficiency of heat exchangers.
TABLE 2 ______________________________________ Typical operation limits of heat exchanger fouling resistance Tube-side liquid.backslash. Two-phase state of Cell-side liquid Vapor Liquid Vapor and liquid ______________________________________ Vapor 3.9 5.1 6.0 Liquid 5.1 6.7 7.9 Two-phase state of 4.8 5.1 6.5 vapor and liquid ______________________________________
With a conventional method for monitoring the fouling resistance of the heat exchanger due to the impurities contained in water, the deposit formation conditions on the heat-transfer surface of a heat exchanger can be visually observed or a surface analysis of the heat exchanger tube can be performed or the deposit weight thereon can be measured. However, with these methods, it is impossible to continuously monitor deposit formation while the system is being operated. In order to monitor the fouling resistance, the operation of the system should be shut down.
In addition, in order to overcome the above-described problems, an apparatus is disclosed for monitoring a deposit build-up by inserting a thermocouple in the heat-transfer surface of the heat exchanger. FIG. 1 illustrates a conventional apparatus for checking deposit formation on the heat-transfer surface of the heat exchanger. As shown therein, in an inlet of the apparatus an inlet temperature sensor 1 and an outlet temperature sensor 2 are installed for measuring the temperature of the inlet and output portions of the apparatus. A temperature sensor (thermocouple, Pt-100.OMEGA., thermister) 6 is inserted in the heat-transfer surface 5 of the heat exchanger disposed within an outer circumferential surface of the heating element 4, which generates heat by power supplied from a power supply unit 1, and then the temperature variation of the heat-transfer surface is measured, thus checking the fouling resistance of the heat exchanger.
As shown in FIG. 1, in the conventional apparatus for monitoring a deposit formation an inlet temperature sensor and an outlet temperature sensor 2 are disposed in inlet and outlet portions of the apparatus for measuring water temperature. A temperature sensor [(thermocouple), Pt-100.OMEGA., thermister] 6 is inserted in a heat-transfer surface 5 of a heat exchanger on an outer circumferential surface of a heating element 4 which generates heat by electric power supplied from a power supply unit 3, so that a temperature variation of the heat-transfer surface of the heat exchanger is measured and the fouling resistance of the heat exchanger is monitored.
As shown in FIG. 1, the heat generated by the heating element 4 is heat-exchanged with the cooling water flowing through the heat-transfer surface 5 of the heat exchanger. If scales having low heat-transfer coefficients are formed on the heat-transfer surface 5, the heat transfer rate of the heat-transfer surface 5 is inhibited, and the heat is not transferred to the cooling water, thus increasing the temperature of the heat-transfer surface 5. As the thickness of the scale is increased, the heat transfered to the cooling water is decreased, and the amount of heat insulated by the scale is gradually increased, whereby the temperature of the heat-transfer surface 5 is increased. At this time, the temperature variation of the heat-transfer surface 5 is directly influenced by the thickness of the scales which inhibits a heat-transfer, namely, by the fouling resistance of the heat-transfer surface 5. The temperature sensor 6 is installed on the heat-transfer surface 5 based on the condition that the temperature of the heat-transfer surface 5 is varied by the fouling resistance of the heat-transfer surface 5, thus measuring the temperature variation of the heat-transfer surface 5, so that the fouling resistance of the heat-transfer surface of the heat exchanger is measured.
However, the conventional fouling resistance monitoring method has the following problems.
First, the conventional fouling monitoring method which uses the temperature sensor 6 installed within the heat-transfer surface 5 is capable of detecting an increase of fouling resistance only when great amounts of scale are formed on the heat-transfer surface. When a small amount of scale is formed on the heat-transfer surface 5, it is impossible to check the temperature variation of the heat-transfer surface 5, and it is impossible to accurately monitor the fouling resistance thereon.
Second, since the scales are not uniformly formed on the heat-transfer surface, an erroneous measured fouling resistance, based on the position of the temperature sensor 6, may be measured.
Factors which cause a fouling resistance of the heat exchanger include the condition of the heat-transfer surface, the flow rate of water, and the water quality. Since the above-described factors are not uniformly applied to the whole heat-transfer surface, the scales are not uniformly formed on the heat-transfer surface. Namely, the scales formed on the heat-transfer surface may be detached therefrom when water flows at high speed, and the thickness of the scales may be increased when water flows at low speed. Under an experiment conducted therefor, the scales were not uniformly formed on the heat-transfer surface. If the fouling resistance is computed based on the erroneously measured temperature at only one point, the fouling resistance may become a factor which causes a serious problem.
FIGS. 2A through 2C illustrate operational principles with respect to a conventional fouling monitoring apparatus. FIG. 2A illustrates a case where the heat-transfer surface 5 is not fouled, and FIG. 2B illustrates a case where scales are uniformly formed on the heat-transfer surface 5. In this case, the temperature measured by the temperature sensor 6 may be considered as a representative temperature of the heat-transfer surface 5. However, as shown in FIG. 2C, if the scales are partially formed on the heat-transfer surface, the temperatures measured in accordance with the installation position of the temperature sensor 6 are different from each other. Therefore, there may be serious errors in the fouling resistance calculated by the measured values.